Automotive heat exchanger cores, such as aluminum radiator cores, are produced by heating a core assembly to bond the air centers to the cooling tubes. Bonding of the air centers results when the core assembly is placed in a vacuum furnace and the cladding material on the tubes melts and brazes the air centers to the tubes. For maximum heat transfer and radiator efficiency, the bonds should result evenly along the width of the cooling tubes where the air centers contact the cooling tubes. However, problems can occur during the manufacturing process that lead to missing bonds, or bonds that are only partially formed. These problems could be due to variations in furnace thermal controls, material inconsistencies, tube and air-center forming problems upstream from the brazing process, etc.
Samples of radiator cores are manually inspected after the brazing process to verify that sufficient bonding did occur. This destructive inspection method requires the sawing or sectioning of cores. The sectioned core is then spread apart for visual inspection. A poor bond is evidenced by the lack of a fillet or ridge of bonding material on the cooling tube. A batch of radiators is determined to be acceptable or unacceptable dependent on the percentage of poor bonds of the destroyed samples. As a destructive inspection technique, the procedure is not only costly, because cores are destroyed, but it is also not reliable. As a manual inspection method, this current procedure is time consuming and labor intensive.
Non-destructive vibration techniques have been known to sample other types of structural members. Exemplary of this technique is U.S. Pat. No. 3,355,933, issued Dec. 5, 1967 to Rowe. The patent discloses a vibration apparatus for testing articles wherein the article is vibrated and a determination of the natural frequency of the article is made by sensing sound waves. The natural frequency is indicative of the relative grade of such articles. Different articles vibrating in identical modes have the same number of nodes of vibration and the same number of antinodes. Therefore, the frequency response in the occurrence at the natural frequencies is compared to known grades to determine which grade being tested the article falls in.
U.S. Pat. No. 4,287,766, issued Sep. 8, 1981 in the name of Ensminger discloses a method of detection of unacceptable solder joints by applying an acoustic vibration over the range of frequencies of the solder joints and observing the acoustic impedance of the joint. The frequency response is compared to a standard spectral response of known acceptable and unacceptable soldered joints which is used to determine whether the solder joint is acceptable or unacceptable. The occurrence of resonant frequencies and the amplitude thereof are compared to the standards for a qualitative determination of acceptability.